In recent years, water-absorbent resins are widely used as component materials of sanitary materials (e.g. disposable diapers, sanitary napkins, incontinent pads) for the purpose of causing the water-absorbent resins to absorb aqueous liquids such as body fluids.
Known examples of the above water-absorbent resins include: partially-neutralized and crosslinked poly(acrylic acids); hydrolyzed copolymers of starch-acrylonitrile; neutralized graft polymers of starch-acrylic acid; saponified copolymers of vinyl acetate-acrylic acid ester; hydrolyzed copolymers of acrylonitrile or acrylamide, or crosslinked polymers of these hydrolyzed copolymers; crosslinked carboxymethyl cellulose; crosslinked copolymers of 2-acrylamido-2-methylpropanesulfonic acid (AMPS); crosslinked poly(ethylene oxide); crosslinked poly(allylamine); and crosslinked polyethylenimine.
Examples of properties, which the above water-absorbent resins should have, include excellent properties when contacting with aqueous liquids (e.g. body fluids), such as: water absorption quantity; absorption rate; liquid permeability; gel strength of swollen gel; and suction force to suck up water from base materials containing the aqueous liquids.
The relations between these properties do not necessarily show positive correlations. For example, there has been a tendency such that, as the absorption properties without load become higher, those under load unfavorably decrease.
Arts in which surface layers of the water-absorbent resins are crosslinked, which are called surface-crosslinking treatment arts, are known as processes for well-balanced improvement of the properties of the water-absorbent resins.
As crosslinking agents used for the surface-crosslinking treatment, there are known such as polyhydric alcohols, polyglycidyl ethers, haloepoxy compounds, polyaldehydes, polyamines, and polyvalent metal salts. As processes for crosslinking the surface layers of the water-absorbent resins with these crosslinking agents, there are known the following representative processes such as: a process in which a surface-crosslink-treating agent and the water-absorbent resin are mixed together and then heated, wherein the surface-crosslink-treating agent is prepared by dissolving the crosslinking agent into water and a hydrophilic organic solvent (e.g. refer to patent documents 1 to 3 below); and a process in which the water-absorbent resin is dispersed into a mixed solvent of water and the hydrophilic organic solvent, and then the crosslinking agent is added to the resultant dispersion to carry out their reaction (e.g. refer to patent document 4 below).
On the other hand, the surface-crosslinking treatment is desired to be carried out without any hydrophilic organic solvent, for example, from the viewpoint of: problems of environmental contamination due to waste liquids and/or waste gases discharged during the production; and use circumstances such that the water-absorbent resin is used for the sanitary materials which contact directly with human bodies.
However, in the case where the water-absorbent resin is produced by using water as the only solvent without the hydrophilic organic solvent when carrying out the surface-crosslinking treatment (e.g. refer to patent document 5 below), then there have been problems in that the properties, particularly, absorption properties, of the water-absorbent resin are unfavorably deteriorated when compared with the case where the hydrophilic organic solvent is used.
Furthermore, hitherto, such as production conditions of the water-absorbent resins themselves and conditions of their surface treatment (as post-processing) have been studied, and many water-absorbent resins have been designed or produced with attention directed to the aforementioned properties (e.g. water absorption quantity, absorption rate, liquid permeability, gel strength, suction force). However, from the viewpoint of practical uses for absorption of excreta and blood (e.g. diapers), it has recently been being found that there is a case where the absorption performances cannot be exercised on the desired high levels by merely satisfying these properties. In other words, as to water-absorbent resins used in absorbent articles for absorption of excreta and blood, there have been problems in that the design or production of the optimum water-absorbent resin has not yet succeeded.